The present invention relates to a process for manufacturing integrated devices which have improved connections between the pins and the semiconductor material chip which integrates electronic components.
As is known, in integrated devices the semiconductor material chip which integrates electronic components is connected to the pins which protrude externally from the housing (package) of the device by means of wires which are soldered on appropriate pads of the chip at one end and to an end of the pins at the other.
It is also known that said wires are currently made of gold or aluminum (or of alloys with a prevalence of gold and aluminum). In particular, gold is the preferred material, by virtue of its characteristics of ductility, workability, conduction etc. which make it the most suitable for connections. However, gold wires cannot be used if they must carry high currents (power integrated circuits). As the current rises, it is in fact necessary to provide wires with a larger diameter, but gold wires with a diameter of more than 50 .mu.m would have an excessively high cost.
Consequently, when the integrated device is a power type device and the wires (or at least some of them) must therefore be able to carry high currents, aluminum is used, since it is easily workable, ductile, has good conductivity etc. and allows the execution of wires with a larger diameter (e.g. 0.3-0.4 mm, which are able to carry a very high current density, equal to 10.sup.6 A/sq. cm.).
Consequently, when the integrated circuit comprises only low-power components (for "signal" control), gold wires are preferably used for the connections, whereas aluminum wires are used in the case of power integrated circuits.
The problems arise with integrated circuits in which a single device integrates both low-power "signal" components and power components. In this case the presence of power components prevents the use of gold wires, so that aluminum wires are currently used.
However, the use of aluminum for both power and signal connections is disadvantageous. Aluminum wires in fact require larger soldering areas with respect to gold, since the soldering technique used for aluminum ("wedge" soldering) requires a larger area than the technique used for gold on the chip ("ball" soldering). Consequently, in the case of integration of both power and signal components in the same device, wires of different diameters are used. However, aluminum wires cannot be produced with small dimensions as is instead possible with gold. Thin aluminum in fact is not sufficiently ductile and breaks easily, so that attempts made with small-diameter (2 mil) aluminum wires have been found to be scarcely reliable.
Consequently, the devices in which aluminum wires of different diameters depending on the currents to be conducted are provided require in any case large areas on the chip for the execution of contact pads with dimensions adequate for the diameter of the wires used and to the soldering technique employed. The problem affects in particular integrated devices which have a large number of pins, in which chips with large surfaces must currently be provided in order to have enough room for the soldering of the connections. Another disadvantage of this technique is the need to use different soldering machines according to the diameter of the wire to be soldered.
It is also known to provide connections formed by a plurality of wires arranged in parallel "multi-wire" technique). By means of this technique it is possible to use gold wires for the conduction of higher currents (the maximum current density is in fact the sum of the current densities of the individual wires connected in parallel). However, even this solution is not free from disadvantages which are linked to the high consumption of wire and most of all to the scarce possibilities of checking the efficiency of multiple-wire connections. Current tests are in fact incapable of distinguishing "good" parts, in which all the wires of each connection are present and unbroken, from defective parts in which one or more wires are missing or in which some wires are weakened (have a reduced cross section). In particular, connections with more than three wires have been found to be impossible to check. It has furthermore been observed that if one of the wires of a connection is broken the other wires of the same connection are also more easily subject to breakage. Due to this reason, when high reliability is required, this technique is generally applied with two, at the most three wires per connection.